A melt-blown method is a melt spinning method for obtaining a fiber assembly from fibers extruded by melting a thermoplastic raw resin. This method employs a melt spinning device having a row of nozzle holes disposed in a row at intervals in a width direction in order to form the fibers by blowing a melted raw resin, and a pair of slits disposed on both sides of the row of the nozzle holes in order to blow hot air. Then, the blown hot air comes into direct contact with melted resin blown from the row of the nozzle holes. Ultrafine fibers are produced while fine fiber forming is performed by a force to crush blown melted resin. The ultrafine fibers are blown to a fiber collecting unit disposed to face the row of the nozzle holes, and are integrated so as to produce an ultrafine fiber assembly.
In a production device of this ultrafine fiber assembly various schemes are introduced in order to efficiently perform fine fiber forming on the fibers. For example, in a device disclosed in PTL 1, a slit for blowing second hot air is disposed on both sides of the row of the nozzle holes for blowing hot air. The blown second hot air is introduced so as to join a distal end of the row of the nozzle holes. The ultrafine fibers are produced while fine fiber forming is performed. In addition, PTL 2 discloses a method for obtaining the ultrafine fibers by disposing a second heating device independent of a heating device for melting a thermoplastic resin, immediately below a spinning nozzle.